A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In current lithographic apparatuses, commonly refractive optics comprising, e.g., a projection lens are used to project a patterned beam onto the substrate. Present developments tend to a reduction of wavelength to achieve a higher resolution and smaller line widths of the patterns to be projected onto the substrate. In the last years, many developments have taken place to construct a lithographic apparatus making use of radiation in the extreme UV (EUV) wavelength range, e.g. making use of 13.5 nm wavelength. At such a wavelength, instead of refractive optics, reflective optics comprising, e.g., projection mirrors are used, as common refractive lenses tend to absorb a significant portion of EUV radiation.
A projection system using reflective optics can pose different design constraints on the lithographic apparatus. As an example, reflective optics are significantly less tolerant to disturbance by vibrations, as compared to refractive optics. This is because a displacement of a mirror in a reflective projection system will have, in general, a larger effect on image quality and accuracy, than a displacement of a lens in a refractive projection system. Such displacement may result in a displacement of the image on a surface of the substrate, a focus error or any other projection error.